#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=3e5+1000;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
int main(void){
   cin.tie(0);std::ios::sync_with_stdio(false);
   LL bl=0;LL wh=0;
   LL n;cin>>n;
   for(LL i=1;i<=n;i++){
       LL x;cin>>x;
       if(x%2==0){
          bl+=x/2;wh+=x/2;
       }
       else{
          bl+=x/2;wh+=x/2;
          if(i&1) bl++;
          else wh++;
       }
   }
   cout<<min(wh,bl)<<"\n";
   return 0;
}
